This invention relates generally to semiconductor devices, and more particularly to an equipment transport mechanism used in the assembly of semiconductor devices.
In the fabrication of many semiconductor devices, a chip or die is mechanically and electrically connected to a leadframe prior to sealing the device in a protective package. Electrical contact is made between conductive pads on the active face of the chip and external leads of the leadframe by a thin metal wire. Several individual leadframes are formed in each leadframe strip. The strip moves as a unit through the various stages of the packaging process, and thus provides a basic component for automated assembly of packaged semiconductor devices.
In conventional packaging processes, a chip is attached to a mount or a die paddle centrally located on the leadframe by an adhesive or alloy, using automated die bonding equipment. Next, bond wires are connected between conductive bond pads on the chip and the lead fingers on the leadframe, using an automated wire bonder. The chips, bond wires, and inner lead fingers are encapsulated in plastic molding compound while the leadframes are in strip format. A trim and form operation separates the individual packaged devices on the leadframe strip, and bends the outer lead fingers into proper lead configuration.
Leadframe strips are stacked, stored, and transported between most of these operations in magazine type carriers, typically an open ended metal container having a series of parallel, horizontal protrusions or shelves inside the carrier which serve to support and separate the leadframe strips. The material is moved on various pieces of automated assembly equipment by way of carriers positioned in an elevator assembly. The elevator moves in a vertical direction to position a selected leadframe strip therein relative to a drive arm and transport blade assembly, whereby the blade assembly is aligned to a particular leadframe strip. The drive arm and blade push the leadframe strip out of the carrier to a location on the assembly equipment where a transport and indexing mechanism takes control of the strip movement. After processing, the complete leadframe strip is inserted into a second carrier on the opposite end of the transport mechanism. Alternately, with linked processing equipment, the transport mechanism continues to guide and move the strip to the next assembly operation.
Leadframes are typically etched or stamped from a thin strip of thermally and electrically conductive metal or alloy, such as a copper alloy. Most frequently, the strips are in the range of 0.004 to 0.020 inches in thickness, 0.5 to 2 inches wide, and 5 to 10 inches long. Rails on either side of the leadframe generally have a low density of patterning, and provide support, as well as the transport contact mechanism for the strip. However, owing to the thickness and low flexural strength, the strips are easily damaged by mechanical contact and force when pushed against an obstructing body.
Precise alignment of the various components in the transport stream is critical to avoid damaging to the leadframe, and destroying the semiconductor device at this late and costly stage in fabrication. It is difficult to precisely control the alignment because the carrier slots are, and must be somewhat larger than the leadframes in order to allow the material to move freely, and to allow for some expansion during the process. After positioning the carrier in line with the transport system, a leadframe strip is ejected from the carrier by a blade whose movement typically is controlled by spring or air pressure, and if the leadframe strip is not perfectly aligned with the transport mechanism, the leadframe rams the transport, and the entire leadframe strip, which in most cases will include the semiconductor chips is destroyed. It is necessary to shut down the equipment and process while an operator removes the leadframe. This is frequently a time consuming operation if the frame is entangled with the transport mechanism.
It would be a significant advantage to the industry if a means of avoiding damage to leadframes caused by imprecise alignment to transport mechanism could be avoided.
It is an object of the invention to provide a means to materially reduce or eliminate damage to leadframe strips caused by misalignment to the transport system.
It is an object of the invention to minimize semiconductor assembly equipment down time by avoiding entanglement of leadframes with transport mechanisms.
It is an object of the invention to provide an inexpensive modification to existing leadframe loading system drive arm and blade assemblages.
It is further an objective of the invention to provide a means for rapid set-up of the drive arm and push blade mechanism, relative to existing leadframe loading systems.
It is an object of the invention to provide a system whereby the amount of force applied to the blade is predetermined, and is adjustable as a function of the strength of the material being moved.
It is an object of the invention to provide a leadframe loading system for a wire bonder, a die bond curing system, a mold loader, a trim and form transport, and/or an integrated assembly system.
It is an object of the invention to provide a magnetic drive loader for trays carrying flex film in strip format.
It is an object of the invention to provide a horizontal push system having a magnetic latch.
The above and other objectives will be met by setting the amount of force applied to a pusher blade to a level below that whereby a leadframe, or other device to be pushed will be damaged, and limiting the amount of force by the size and strength of a magnetic latch attached to the drive arm. The magnet is designed to break away from the steel drive arm when force exceeds a preset value, and stops the material movement before damage to the leadframe occurs. The failure is detected by the assembly equipment computer.
The preset value is determined from the material characteristics of the leadframe, and size and strength of the magnet. Calculated values were verified by physical testing, and a safety margin assigned to insure release of the magnet prior to bending leadframes.